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# ch22 - Electric Force and Electric Charge Chapter Outline...

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Unformatted text preview: Electric Force and Electric Charge Chapter Outline ENGINEERING PHYSICS II Properties of Electric Charges PHY 303L Charging Objects By Induction Charge Conservation Chapter 22: Electric Force and Electric Charge Coulomb’s Law Maxim Tsoi Physics Department, The University of Texas at Austin http://www.ph.utexas.edu/~tsoi 303L: Electric Fields (Ch.22) 303L: Electric Fields (Ch.22) Properties of Electric Charges Properties of Electric Charges Experiments demonstrating the existence of electric forces and charges Two kinds of electric charges • positive and negative • Run a comb through hair • We identify comb attracts bits of paper • Rub an inflated balloon with wool negative charge as that possessed by electrons positive charge as that possessed by protons balloon adheres to a wall • Experiment verifying that there are two types of charges: When materials (e.g., comb, balloon) behave in this (a) A negatively charged (b) A negatively charged way, they are said to be electrified, or to have become rubber rod, suspended by rubber rod is repelled by electrically charged a thread, is attracted to a another positively charged glass charged rubber rod negatively rod • charges of the same sign repel one another and charges with opposite signs attract one another 303L: Electric Fields (Ch.22) 303L: Electric Fields (Ch.22) Properties of Electric Charges Conservation of electrical charge Properties of Electric Charges • When one object is rubbed against another, charge is not created in the process • The electrified state is due to a transfer of charge from one object to another • We now know that electrons are transferred between objects • There are two kinds of charges in nature; charges of opposite sign attract one another and charges of the same sign repel one another Electric charge is always conserved in an isolated system • Total charge in an isolated system is conserved • When a glass rod is rubbed with silk, electrons are Charge is quantized transferred from the glass to the silk. Because of conservation of charge, each electron adds negative charge to the silk, and an equal positive charge is left behind on the rod 303L: Electric Fields (Ch.22) 303L: Electric Fields (Ch.22) 1 Insulators and Conductors Charging Objects Materials classification in terms of the ability of electrons to move by conduction Charging a neutral metallic (conducting) sphere • Conductors (copper, aluminum, silver) materials in which some • Just before contact, the negative rod repels the of electrons can move relatively freely through the material sphere’s electrons, inducing a localized positive charge • Insulators (glass, rubber, wood) materials in which all electrons are bound to atoms and cannot move freely through the material • After contact, electrons from the rod flow onto the sphere, neutralizing the local positive charges • Semiconductors (silicon, germanium) their electrical properties are somewhere between those of insulators and those of conductors • When the rod is removed, the sphere is left with a negative charge 303L: Electric Fields (Ch.22) 303L: Electric Fields (Ch.22) Charging Objects Charging Objects by induction Induction in insulators • Neutral metal (conducting) sphere, with equal • The charged object on the left induces numbers of positive and negative charge charges on the surface of an insulator • The charge on the sphere is redistributed when a charged rubber rod is placed near the sphere • When the sphere is grounded, some of the electrons leave it through the ground wire • A charged comb attracts bits of paper because charges are displaced in the • When the ground connection is removed, the paper. nonuniformly charged sphere is left with excess positive charge • When the rubber rod is moved away, the charges on the sphere redistribute themselves until the sphere’s surface becomes uniformly charged 303L: Electric Fields (Ch.22) 303L: Electric Fields (Ch.22) Coulomb’s Law Coulomb’s Law Coulomb’s experiments (1785) Electric force between two point charges We can express Coulomb’s law as: An electric force has the following properties: • It is directed along a line joining the two particles and is inversely Fe = ke proportional to the square of the separation distance r, between them • It is proportional to the product of the magnitudes of the charges, q1 and q2, of the two particles q1 q2 r2 • Coulomb constant ke = 8.9875×109 Nm2/C2 (ke = 1/4πε0) • Permittivity of free space ε0 = 8.8542 ×10-12 C2/Nm2 • The SI unit of charge is coulomb (C) • It is attractive if the charges are of opposite sign and repulsive if the charges have the same sign 303L: Electric Fields (Ch.22) • The charge of an electron e=1.6×10-19 C • 1C 6.24×1018 electrons 303L: Electric Fields (Ch.22) 2 Coulomb’s Law SUMMARY Vector form Electric Force and Electric Charge The electric force exerted by a charge q1 on a second charge q2 is • Properties of electric charges: • charges of opposite sign attract one another and charges of • total charge in an isolated system is conserved • r qq ˆ F12 = ke 1 2 2 r r charge is quantized the same sign repel one another • Conductors are materials in which electrons move freely. • • Insulators are materials in which electrons do not move freely. ˆ r is a unit vector directed from q1 toward q2 r r F12 = − F21 • Coulomb’s law: • When more than two charges are present, the resultant force on any one of them: 303L: Electric Fields (Ch.22) r qq ˆ F12 = ke 1 2 2 r r • The smallest unit of free charge is the charge of one electron (-e) or proton (+e), where e=1.6×10-19 C r r r r F1 = F21 + F31 + F41 Example 1 303L: Electric Fields (Ch.22) 3 ...
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